Process for recovering inorganic material from deinking sludge

ABSTRACT

A process for recovering inorganic material from a deinking sludge obtained as reject material in the recovery of recycled fiber from waste paper. In accordance with the invention the process comprises the following steps: (a) the proportion of organic material in the form of fiber material in the deinking sludge is 15-40% (dry solids content), (b) the water content of the sludge is adjusted so that the dry solids content becomes 10-35%, step (a) and step (b) being so adapted to each other that the sludge is pumpable, (c) the pressure of the sludge is increased to at least 22 MPa, (d) the temperature is increased to supercritical or near supercritical temperature, (e) a medium containing oxygen is added to the sludge to oxidize all organic material at supercritical temperature and pressure in a reactor, (f) energy released at the oxidation of the organic material is recovered, and (g) the flow of material from which energy has been recovered and which comprises said inorganic material, gases and water, is subjected to material separation for recovery of the inorganic material.

[0001] The present invention relates to a process for continuouslyrecovering inorganic material from a deinking sludge obtained as rejectmaterial in the recovery of recycled fiber from waste paper.

[0002] Such deinking sludge has a dry solids content that is normallyless than 8% prior to dewatering and contains organic material in anamount normally in excess of 35%, the remainder being inorganicmaterial, calculated on the dry solids content.

[0003] The term waste paper refers to paper or cardboard which, afteruse, is recovered in the form of recycled fiber and then, afterslashing, is re-used to produce paper or cardboard or for some otherindustrial purpose. Recycled fiber thus refers to fiber material thathas previously been used in a paper or cardboard product.

[0004] Paper used for manufacturing various types of printed matter,such as newspaper, magazine, LWC and fine paper, contains a plurality ofadditives. The finished printed paper products also contain printingink. The additives comprise primarily fillers, pigments and colorants,but other additives may also be included. The quantities and therelative proportions of the various additives vary depending on the typeof printing paper. The main purpose of the fillers is to increase theopacity of the printing paper. Clay, calcium carbonate, titanium dioxideand/or talcum are generally used as fillers, but other materials may beused. Pigment is included in the coating layer of coated paper orcardboard. Clay and calcium carbonate are mainly used as pigments,either separately or in combination, however, titanium dioxide andtalcum may also be used.

[0005] When recovering recycled fiber from waste paper, the waste paperis subjected to a cleaning process in order to decolour the fibers andremove impurities. The various rejects thus obtained are then combinedto produce a deinking sludge with relatively high water content. Noindustrially practicable method for recovering valuable material fromthis deinking sludge has so far been used or even proposed and, instead,the deinking sludge represents a residual product which, afterdewatering, can be used in the cement and brick-making industry or beincinerated using conventional technology. The ash obtained fromincineration is deposited in suitable locations or used as land fill.Some factories and countries even deposit deinking sludge that has notbeen incinerated, which is a load from the environmental point of view.

[0006] In the article entitled “Supercritical water oxidation of pulpmill sludges” by M. Modell et al, 1991 Engineering Conference, pages393-403, an alternative method to conventional incineration is describedfor treating sludge from pulp mills. The method, generally termed theSCWO process (Super-Critical Water Oxidation), is based oncarbonization-free oxidation of the organic part of the sludge in thepresence of water in a supercritical state. This means that theoxidation of organic material takes place at a temperature and at apressure above the supercritical point in the phase diagram of thewater, i.e. 374° C./22 MPa. Oxidation of organic material at therelatively low temperatures is possible due to the high pressure and thepresence of water as reaction medium. At its supercritical point 374°C./22 MPa and in its supercritical area above this point, the waterbehaves as a supercritical fluid and in this state the supercriticalwater acts as an efficient solvent for organic material and gases. Thearticle only discusses sludges obtained in the conventional manufactureof pulp and thus mentions nothing about the specific deinking sludgeobtained from the recovery of recycled fiber from waste paper, nor doesit mention anything about the use of the method for recovering inorganicmaterial from deinking sludge. An article entitled “Nassoxidation vonDeinkingschlämmen” by U. Hamm and L. Göttsching, Wochenblatt fürPapierfabrikation 1, 1998, pages 15-23 describes results fromexperiments in deoxidizing deinking sludge using the SCWO processmentioned above. It is clear from the article that, even at extremelyhigh oxidation of the organic material in the deinking sludge, aninorganic material is obtained with such low brightness, less than 60%ISO, that it cannot be used as filler. The state of the art discusseddoes not therefore show or intimate the recovery of filler from deinkingsludge for re-use in a paper-manufacturing process making use ofoxidation in accordance with said SCWO process.

[0007] The object of the present invention is to provide a process forrecovering inorganic material from deinking sludge, said material beingof such quality as to be suitable for re-use in the paper-manufacturingprocess.

[0008] The process in accordance with the invention is characterized inthat it comprises the following steps:

[0009] (a) controlling and/or adjusting the proportion of organicmaterial in the form of fiber material in the deinking sludge so that itis or becomes 15-40%, calculated on the dry solids content,

[0010] (b) adjusting the water content of the deinking sludge so thatthe dry solids content becomes 10-35%, step (a) and step (b) being soadjusted to each other that the deinking sludge is pumpable,

[0011] (c) increasing the pressure of the pumpable deinking sludge to atleast 22 MPa,

[0012] (d) increasing the temperature of the pumpable deinking sludge tosupercritical or almost supercritical temperature,

[0013] (e) adding a medium containing oxygen to the deinking sludge in asufficient quantity to oxidize all organic material, which takes placeat supercritical temperature and pressure in a reactor,

[0014] (f) recovering energy released at the oxidation of the organicmaterial in a suitable form, and

[0015] (g) subjecting the flow of material from which energy has beenrecovered and which comprises said inorganic material, gases and water,to material separation comprising recovery of the valuable inorganicmaterial.

[0016] The invention is described further in the following withreference to the drawings.

[0017]FIG. 1 shows a flowchart for a recovery plant for recoveringinorganic material from deinking sludge in accordance with theinvention.

[0018] Deinking sludge from a cleaning process for waste paper ischarged into a storage tank 1 and stirred constantly to preventsedimentation. If the deinking sludge has not been treated in specialdewatering equipment, it normally has a dry solids content below 8%,usually about 5% or lower, the remainder being water. The deinkingsludge normally has an organic material content over 35%, calculated onthe dry solids content, usually within the interval 45-50%, the contentof inorganic material thus being below 65% and 55-50%, respectively.

[0019] The organic material comprises fiber material, printing ink,other colorants, various types of organic additives and binders, thefiber material thus constituting the largest proportion. The fibermaterial consists of fibers and fiber parts, so-called “fines”. Theinorganic material comprises primarily filler, but even other inertmaterials may be included, such as pigment, depending on the type ofpaper or cardboard product constituting the waste paper.

[0020] The deinking sludge is conveyed continuously to a pretreatingplant 2 for suitable pretreatment so that the proportion of fibermaterial of the dry solids content is reduced to a predetermined level.Several different methods are available as suitable pretreatment toachieve such a reduction in the fiber material content, such asflotation, sedimentation, vortex cleaning, screening, centrifuging,biological decomposition (aerobic or anaerobic) of a part of the fibermaterial, converting a part of the fiber material to ethanol throughenzymatic or some other type of hydrolysis followed by fermentation, ora combination of two or more of these technologies, which may beperformed in one or more steps. The pretreatment of the deinking sludgein order to reduce the proportion of fiber material to a predeterminedlevel may also be integrated in the deinking process. In this case noseparate pretreatment plant is required. Reducing the content of fibermaterial so that the amount of organic material remaining is between 15and 40% is important if a process is to be obtained that has acceptableoperating economy. A reduction in the content of organic materialincreases the capacity of a SCWO process plant since deinking sludgewith a higher dry solids content can be used. This is in turn a resultof the COD content being lower, and of better pumpability of thedeinking sludge. Furthermore, the cost of oxygen gas is lower if theratio between organic and inorganic material decreases.

[0021] The deinking sludge treated in this manner is then conveyed to aplant 3 to increase the concentration of dry solids to a value withinthe range 15-35%. This increase in the concentration may be achieved byperforming a suitable treatment that either directly produces thisconcentration interval or first a considerably higher concentrationafter which the deinking sludge is diluted so that a concentrationwithin the interval 15-35% is obtained. Several methods are available assuitable treatment to achieve such an increase of the dry solidscontent, such as sedimentation, vortex cleaning, centrifuging, pressingin a suitable press, or a combination of two or more of thesetechnologies. Thickening of the deinking sludge is necessary in order toreduce the amount of water present during the oxidation performeddown-stream. When the dry solids content is increased, or immediatelythereafter, the deinking sludge may be subjected to washing,particularly if the salt content in the sludge is too high. Metal ionsare removed during this wash and may form metal oxides in the subsequentoxidation which may discolour the inorganic material recovered, therebyreducing its brightness. In order to avoid salt deposits, so-calledencrustations, in the plant the salt content should be kept as low aspossible, preferably below 1000 ppm. The deinking sludge must still bepumpable after the increase in dry solids content in order to enablecontinued processing. The two main initial steps must therefore beadjusted to each other so that the deinking sludge leaving the plant 3is in pumpable state and also has a COD content of 40-200 g/l,preferably 60-150 g/l. Dispersion methods may be used to increase thepumpability of the deinking sludge. The pumpable deinking sludge isconveyed to a storage tank 4 provided with an arrangement to homogenizethe deinking sludge so that a uniform concentration is maintained in thestorage tank 4 and so that particularly the fiber material isdisintegrated and/or prevented from forming large particles of fibersdue to the action of a pump. The deinking sludge is pumped from thestorage tank 4 by means of a pump arrangement comprising a high-pressurepump 5 that give the deinking sludge an operating pressure of at least22 MPa.

[0022] The deinking sludge is then conveyed to a heating device from itis discharged with a suitable temperature. This temperature ispreferably at most 425° C. The heating device comprises a heat exchanger6 which functions throughout the continuous operation, and thereafter apreheater 7 which may be run on natural gas, for instance, and isconnected if the heat exchanger has insufficient capacity, such as whenthe recovery plant is being started up.

[0023] The deinking sludge, including the water in supercritical or nearsupercritical state, is conveyed to a reactor 8 which may consist of avertical reaction vessel, for instance, the deinking sludge beingsupplied at the top of the reactor. The reactor may alternatively be inthe form of an elongate reaction pipe which may be bent, e.g. in a flatserpentine loop or in a circular spiral. (The SCWO process plant maynaturally use a reactor of some other shape.) A medium containingoxygen, preferably liquid oxygen, is simultaneously fed in at the top ofreactor 8 through a pipe 9 in order to be intimately mixed with thedeinking sludge. The oxygen oxidizes the organic material in thedeinking sludge and the exothermal reaction thus obtained gives adesired increase in temperature in the reactor. The reaction iscontrolled so that the temperature in the reactor is kept above 500° C.The temperature is preferably kept within the interval 500-650° C., mostpreferably within the interval 550-600° C. The amount of oxygen suppliedis adjusted to the requirement measured, which lies within said CODcontent 40-200 g/l. Under these temperature and pressure conditions, andadjustment of the oxygen supply, a complete oxidation of all organicmaterial in the deinking sludge is achieved. Operation at a controlledtemperature as stated above, preferably below 650° C., ensures that thestructure of the inorganic material is not destroyed. During thecontinuous operation a flow of material containing solid and dissolvedinorganic material, gases and water is withdrawn from the reactor. Thisflow of material is caused to pass the heat exchanger 6 in order totransfer a part of its heat content to the ingoing flow of deinkingsludge. The outgoing flow of material then passes a steam generator 10(or some other system for recovering energy), where the remaining energycan be recovered, a final cooler and then a pressure-reducing device.

[0024] The material flow cooled and pressure-relieved in this way isfinally conveyed to a separation plant comprising a separator 12 forseparating the gas from the aqueous inorganic material. The gas phase iscarried through a pipe 13 out into the atmosphere or is collected insuitable manner to make use of its constituents, e.g. carbon dioxide,while the aqueous inorganic material in the form of a suspension iswithdrawn through a pipe 14. If necessary, the suspension may bethickened in suitable manner, e.g. in a sedimentation tank or screeningdevice (not shown). Washing may also be performed either before or afterthe thickening in order to remove any residual soluble compounds. Theseparated water is conveyed to recipients or is returned to one of theprocess steps in the mill.

[0025] The properties of the inorganic material thus recovered aredetermined. If the brightness is found to be about 68% ISO or above, theinorganic material can be used directly as filler in a papermanufacturing process, provided the material fulfils the requirementsstipulated for such use. These requirements are that the materialcontributes to the optical and technical printing properties striven forin the paper production concerned, that it has such properties as not togive negative effects in comparison with conventional products on themarket and that, from the resource, environmental and/or energy aspects,it contributes to effectivization of the paper manufacture.

[0026] In order to improve the quality, primarily the brightness, theinorganic material recovered is subjected to a suitable after-treatmentin an after-treatment plant 15. This may be performed, for instance, bycleaning in a special magnetic separator of high-gradient type where themetal oxides formed during oxidation in the reactor, which reducebrightness, are removed. Dispersing agent may be added before thecleaning in order to achieve the best result. The brightness can also beimproved by bleaching with a suitable bleaching agent, e.g. dithionite.A combination of these cleaning methods may also be used to achieve evenhigher brightness. Such an after-treatment can increase the brightnessby 2 or more % ISO. The inorganic material recovered, with or withoutsaid after-treatment, can be used as filler in the manufacture ofnewspaper of various grades, for instance. If subjected toafter-treatment as described above, it can be used as filler in productsdemanding higher brightness, such as magazine, LWC and fine paper.

[0027] If desired, the medium containing oxygen may be supplied at twoor more points along the reaction vessel and/or before this. Thepredetermined total amount of medium containing oxygen can thus bedistributed between these points in equal or different parts.

[0028] Pigment has a smaller average particle size than the filler. If alarger proportion of the waste paper comes from coated paper orcardboard it may be profitable, using suitable equipment, to divide orfraction the inorganic material into a first part having large particlesize and a second part having smaller particle size, which applies topigment. The first part then constitutes filler and the second partpigment which can be used in a coating paste.

[0029] Tests have shown that the inorganic material recovered inaccordance with the invention is re-usable as filler in a papermanufacturing process. Tests have also shown that the re-use of fillerrecovered in accordance with the invention in printing paper givessatisfactory results as regards physical, optical and printingproperties.

[0030] The following is an account of experiments performed inaccordance with the invention.

[0031] Deinking sludge from a paper mill with a conventional deinkingplant for waste paper was treated on a pilot scale in a flotation plantin order to reduce the proportion of fiber material in the deinkingsludge. In conjunction with the flotation treatment some of the saltswere also washed out of the deinking sludge. The proportion of filler,calculated on the dry solids content, was increased by the flotationtreatment to about 68% (32% organic) from about 55% originally. Theflotation process was not completely optimized during this experiment.With an optimized flotation process it should be possible to reach afiller content of about 80%, calculated on the dry solids content. Thisvalue was achieved in another waste paper factory in experiments on asmaller scale. After the flotation treatment the dry solids content ofthe deinking sludge was increased to about 20% in a dewatering device.

[0032] The above-mentioned dewatered deinking sludge was then treated ina SCWO process plant having a capacity of 250 l/h. The oxidation tookplace at a temperature of between 550° C. and 580° C., a pressure of 25MPa and a dwell time in the reactor of about 1 minute. The filler in thematerial flow leaving the SCWO process plant had a brightness of 72-73%ISO. The filler from this experiment was then used in a full-scaleexperiment in a newspaper machine where it was mixed into the paper sothat an increased filler content was obtained. Evaluation of thisexperiment showed that the properties of the filler recovered were asgood as those of fresh filler.

[0033] In another experiment in the same SCWO process plant deinkingsludge from another paper mill producing SC paper from waste paper wasused. After the SCWO process treatment a filler having a brightness of78-79% ISO was obtained.

[0034] Experiments were also performed in the above-mentioned SCWOprocess plant using deinking sludge to which extra filler in the form ofclay and chalk had been added so that a filler content of 18% and afiber content of 5.5% were attained, giving a total dry solids contentof 23.5% and an organic material content of 23% calculated on dry solidscontent, in the deinking sludge supplied to the SCWO process plant. Thisdeinking sludge could be run in the SCWO process plant withoutdifficulty.

[0035] Samples of the same type of recovered filler as were used in thefull-scale experiment in the paper machine were treated in a magneticseparator of high-gradient type. This treatment gave increasedbrightness of up to 3.5%-units ISO.

[0036] Experiments were also performed with the above-mentioned filler,with dithionite bleaching. This gave increased brightness of 1-2%-unitsISO.

1. A process for continuously recovering inorganic material from adeinking sludge obtained as reject material in the recovery of recycledfiber from waste paper, characterized in that it comprises the followingsteps: (a) controlling and/or adjusting the proportion of organicmaterial in the form of fiber material in the deinking sludge so that itis or becomes 15-40%, calculated on the dry solids content, (b)adjusting the water content of the deinking sludge so that the drysolids content becomes 10-35%, step (a) and step (b) being so adjustedto each other that the deinking sludge is pumpable, (c) increasing thepressure of the pumpable deinking sludge to at least 22 MPa, (d)increasing the temperature of the pumpable deinking sludge tosupercritical or almost supercritical temperature, (e) adding a mediumcontaining oxygen to the deinking sludge in a sufficient quantity tooxidize all organic material, which takes place at supercriticaltemperature and pressure in a reactor, (f) recovering energy released atthe oxidation of the organic material in a suitable form, and (g)subjecting the flow of material from which energy has been recovered andwhich comprises said inorganic material, gases and water, to materialseparation comprising recovery of the valuable inorganic material.
 2. Aprocess as claimed in claim 1, characterized in that the quantity oforganic material in said step (a) is 15-30%.
 3. A process as claimed inclaim 1 or claim 2, characterized in that said step (a) and step (b) areadapted to each other so that, after said step (b) the deinking sludgehas a COD content within the interval 40-200 g/l, preferably 60-150 g/l.4. A process as claimed in any one of claims 1-3, characterized in thatin said step (d) the temperature is increased to at most 425° C.
 5. Aprocess as claimed in any one of claims 1-4, characterized in that thetemperature in the reactor after said step (e) is at most 650° C.
 6. Aprocess as claimed in claim 5, characterized in that the temperature inthe reactor is between 500° C. and 600° C.
 7. A process as claimed inany one of claims 1-6, characterized in that gaseous reaction productsare separated from the flow of material in said step (g) and arereleased into the atmosphere or collected.
 8. A process as claimed inclaim 7, characterized in that carbon dioxide is recovered from thegases collected.
 9. A process as claimed in any one of claims 1-8,characterized in that the deinking sludge is subjected to washing priorto said step (c).
 10. A process as claimed in claim 9 characterized inthat the deinking sludge has a salt content of at most 1000 ppm aftersaid washing.
 11. A process as claimed in any one of claims 1-10,characteriz d in that the oxidation time in the reactor is about 10minutes or less, preferably about 5 minutes or less and most preferablyabout 1 minute.
 12. A process as claimed in any one of claims 1-11,characterized in that the fiber material is removed from the deinkingsludge in order to achieve a content within the interval in step (a) bymeans of flotation, sedimentation, vortex cleaning, screening,centrifuging or a combination of two or more of these techniques.
 13. Aprocess as claimed in any one of claims 1-11, characterized in that thefiber material is removed from the deinking sludge in order to achieve acontent within the interval in step (a) by means of flotation,sedimentation, vortex cleaning, screening, centrifuging, or acombination of two or more of these techniques being integrated into thedeinking process.
 14. A process as claimed in claim 12 or claim 13,characterized in that recycled fiber is recovered from the fibermaterial removed.
 15. A process as claimed in any one of claims 1-11,characterized in that the fiber material is removed from the deinkingsludge in order to achieve a content within the interval in step (a) bymeans of biological decomposition (aerobic or anaerobic) of a part ofthe fiber material, converting a part of the fiber material to ethanolthrough enzymatic or some other type of hydrolysis followed byfermentation, or a combination of two or more of these techniques.
 16. Aprocess as claimed in any one of claims 1-15, characterized in that thewater content is adjusted in step (b) by means of sedimentation, vortexcleaning, centrifuging, pressing in a suitable press, or by acombination of two or more of these techniques.
 17. A process as claimedin any one of claims 1-16, characterized in that the inorganic materialobtained in step (g) has a brightness of at least 68% ISO.
 18. A processas claimed in any one of claims 1-17, characterized in that theinorganic material obtained in step (g) is subjected to after-treatmentin order to increase the brightness by 2 or more % ISO.
 19. A process asclaimed in claim 18, characterized in that the after-treatment comprisescleaning to remove brightness-reducing metal oxides and/or bleachingwith a bleaching agent.
 20. A process as claimed in any one of claims1-19, characterized in that the valuable inorganic material recoveredalso contains pigment having a smaller particle size than the particlesize of the filler, the inorganic material being divided into a fillerfraction and a pigment fraction.
 21. A process as claimed in any one ofclaims 1-20, characterized in that the energy in step (f) is recoveredin the form of vapour and/or hot water.
 22. A process as claimed in anyone of claims 1-21, characterized in that the incoming deinking sludgehas a dry solids content that is normally less than 8% prior todewatering and contains organic material in a quantity that is normallyover 35%, the remainder being inorganic material, calculated on the drysolids content.